1. Field of the Invention
This invention relates to multimode optical fiber.
2. Description of the Prior Art
Fiber-based lightwave communication systems are expected to play an important role in voice and data transmission in the near future. Because of this large application potential, improvements in the performance characteristics of fiber for such systems are of considerable interest and may result in substantial economic benefits.
Optical fiber for use in communication systems can be either single mode fiber or multimode fiber. This application is concerned only with the latter. Such multimode fiber comprises a core, to which essentially all of the signal energy is confined, and a clad surrounding the core. Frequently a barrier layer is interposed between core and clad. See for instance Tingye Li, Proceedings of the IEEE, Vol. 68(10), pages 1175-1180, 1980, and P. B. O'Connor et al, American Ceramics Society Bulletin, Vol 55(5), pages 513-519, 1976.
The electromagnetic signal propagating in a multimode fiber, herein often referred to as "light", is usually adversely affected by at least two mechanisms: attenuation and dispersion.
The attenuation per unit length of fiber as a function of wavelength is often referred to as the loss spectrum of the fiber. It typically is comprised of several essentially irreducible contributions, e.g., Rayleigh scattering and absorption in the core matrix, and contributions that are in principle reducible. The latter are due, for instance, to absorption of the signal radiation by impurities in the fiber, to scattering from gross fiber defects, or to variations in fiber parameters, such as, for instance, variations in core diameter of refractive index profile. Current manufacturing practices typically succeed in producing fiber of sufficient uniformity to make attenuation due to the last-mentioned mechanisms negligible. And great efforts have been expended world-wide to reduce losses due to absorption by impurities in the fiber. These efforts have succeeded to the point where now fibers can be produced that have less than 1 db/km loss at appropriate wavelengths, such as for instance at 1.3 .mu.m. In particular, loss due to OH can now be essentially eliminated.
Signal dispersion in multimode fiber is due to chromatic dispersion and to mode dispersion. Since this application is concerned substantially only with the latter, I will not discuss chromatic dispersion herein. The effect of dispersion is expressed in terms of the fiber bandwidth per unit length, or equivalently, in terms of a maximum bit rate per unit length.
In multimode fiber the signal's energy is typically distributed over many modes, and each mode has its own wave propagation path and power distribution within the fiber core. In a fiber having a core of radially uniform refractive index these propagation paths result in substantially different propagation times for the different modes, leading to severe modal dispersion. However, it is possible to greatly reduce mode dispersion by designing the fiber to have an appropriate radially varying refractive index in the core region. Such graded index lightguides can have relatively high bandwidth over a limited range of wavelenghts centered around the design wavelength for which the profile is optimized. (See for instance Tingye Li, op. cit.)
It is typical current practice in high-capacity long-haul optical communication systems to use fiber that has as little loss as possible at the design wavelength, in addition to having the largest possible bandwidth at that wavelength.
Despite the relatively high bandwidths achievable with graded index fibers the repeater spacings in long-haul fiber communication systems are often bandwidth-determined. Thus techniques for increasing the bandwidth of multimode fiberguide are of considerable interest, since this would often, for instance, permit an increase in repeater spacing. Furthermore, it would be desirable to increase the extent of the wavelength regime throughout which a multimode fiber has high bandwidth, since this would allow for instance operation of a system at two or more signal wavelengths, as for instance in a wavelength multiplexed system, or permit the upgrading of a system to operate at a different and more advantageous wavelength for which, for instance, the technology was not available at the time of installation of the system.